HCN4-like immunoreactivity in rat retinal ganglion cells

Antisera directed against hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels bind to somata in the ganglion cell layer of rat and rabbit retinas, and mRNA for different HCN channel isoforms has been detected in the ganglion cell layer of mouse retina. However, previous studies neither provided evidence that any of the somata are ganglion cells (as opposed to displaced amacrine cells) nor quantified these cells. We therefore tested whether isoform-specific anti-HCN channel antisera bind to ganglion cells labeled by retrograde transport of fluorophore-coupled dextran. In flat-mounted adult rat retinas, the number of dextran-backfilled ganglion cells agreed with cell densities reported in previous studies, and anti-HCN4 antisera bound to the somata of approximately 40% of these cells. The diameter of these somata ranged from 7 to 30 microm. Consistent with localization to cell membranes, the immunoreactivity formed a thin line that circumscribed individual somata. Optic fiber layer axon fascicles, and the proximal dendrites of some ganglion cells, also displayed binding of anti-HCN4 antisera. These results suggest that the response of some mammalian retinal ganglion cells to hyperpolarization may be modulated by changes in intracellular cAMP levels, and could thus be more complex than expected from previous voltage and current recordings.